Process for determining the coefficient of friction mμ

ABSTRACT

A method for determining the coefficient of friction μ between the wheel and the road is described. Assuming a moving vehicle, the brake pressure on a non-driven wheel (measuring wheel) is increased until a tendency to lock occurs. At the same time, the driving torque is increased so that the braking force of the measuring wheel and the increased driving force of the driving wheel on the same vehicle side equal each other. Additionally, an increased driving force of the second driving wheel must be removed by braking.

BACKGROUND OF THE INVENTION

From DE-OS 37 05 983, a method is known for equipping a vehicle with ananti-locking brake system (ABS) and an automatic slip control (ASR).This reference, however, makes a method known by which only thecoefficient of friction is determined and indicated in order to promptthe driver to adopt a mode of driving behavior which is adapted to theprevailing coefficient of friction. In the reference, the coefficient offriction is only determined during braking or acceleration of thevehicle.

SUMMARY OF THE INVENTION

In the present invention, determination of the current coefficient offriction between the tire and the road is desired at all times, not onlyduring braking or acceleration, in order to be able to give earlywarning to the driver, for example at low friction values (snow, ice,wetness, etc.). In addition to the warning or in place of it, a visualor audible indication of the approximate coefficient of friction ispossible.

If, in a front-wheel drive vehicle, for example, the right-hand frontwheel VR is driven and the right-hand rear wheel HR is braked to thesame degree, then there will be no change in the overall drivingcondition: the vehicle will continue to roll at the same speed V, but atensioning condition will occur between front and rear wheels, and thelateral guiding forces will decrease on these wheels.

If the driving condition of the front wheel VR and the braking conditionof the rear wheel HR are further increased (assuming that the output ofthe engine is sufficient), then the mutual tensioning condition willreach a value at which the front wheel VR will tend to spin and the rearwheel HR will tend to lock. This value A_(H) has the following relationto the coefficient of road friction μ:

    A.sub.H =μ×N.sub.H /2

(assuming there are equal loading forces N_(H) /2 at the rear wheels).

If N_(H) /2 is known (by way of approximation, or to be on the safeside, an unladen vehicle is assumed), then the coefficient of frictioncan be calculated from the value A_(H). The force A_(H), in turn, isproportional to the braking force at the brake callipers or the pressurein the brake callipers. If this pressure is now created by being pulsedup from a constant pressure source via solenoid valves, then when takinginto account the pressure-volume characteristic curve of the brakingsystem, the sum of the accordingly corrected pulse times of the solenoidvalve is a measure for the pressure in the brake callipers, and thusapproximately proportional to the coefficient of road friction μ. Apressure increase is also possible via a ramp function, e.g. throughseverely throttled solenoid valves in permanently open position. Theword "continuously" is used to indicate that the pressure increaseshould not be sudden.

The invention is to be mainly applied where ABS/ASR are used, since thenonly a few additional parts will be needed.

Since, with an increase in engine torque without braking, the seconddriven wheel would be accelerated and would create a yawing momentaround the normal axis of the vehicle, it must be held at the speedprevailing when μ was first determined.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be explained on the basis of thedrawings.

The figures show the following:

FIG. 1. A well-known ABS/ASR, which has been extended according to anembodiment of the invention.

FIG. 2 A second ABS/ASR, which has also been extended according to afurther embodiment of the invention.

FIG. 3 An extended control circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an ABS and ASR for a front-wheel drive vehicle withdiagonal braking circuit splitting. In the case of the ASR, for thepurpose of brake control, an ABS recirculating pump 1 is set inoperation which creates the required brake pressure for the left frontwheel VL. This pressure is then kept constant via a then switchedchangeover valve 2 and a pressure limiter 3. By means of a 3/3 ABSsolenoid valve 4, the pressure at the brake of the left front wheel VLis then varied. The system also has a pressure-controlled loading valve7 and a second ABS pump 1.

Similarly, in the case of the ASR, the pressure on the brake of theright front wheel VR is varied by valves 8 and 9.

For the purpose of the invention, an additional changeover valve 5 isnow proposed.

If μ is to be determined at constant velocity, then a signal isproduced, for example, by means of a key, which operates the changeovervalves 2 and 5, and switches on pump 1. Via the ABS valves 4 and 6,pressure can now be induced individually to the brakes of the wheels HRand VL by a control device. Pressure is introduced in such a way that onthe one hand the brake pressure on the rear right-hand wheel HR isprogressively pulsed, while at the same time the drive output isincreased so that the driving force of front right-hand the wheel VR andthe braking force of the rear right-hand wheel HR approximatelycompensate each other, and on the other hand the braking force on thefront right-hand wheel VL is increased in such a way that its speed isheld approximately constant.

This control is continued until a tendency to lock occurs at the wheelHR. The number of pulses which were needed to reach this level ofpressure represents a measure for the desired μ value on the right-handside of the vehicle, where as a rule the more slippery track is found(where traffic travels on the right-hand side of the road).

The measuring action of μ can be triggered automatically if the vehicleis moving straight ahead at constant speed. If the driver brakes duringmeasurement, then the measuring action is discontinued.

In the embodiment of FIG. 2, a rear wheel drive vehicle with axisbraking circuit splitting is assumed. Here, the right-hand front wheelVR is used as the measuring wheel for the measurement of μ and brakeduntil the tendency to lock appears, the right-hand rear wheel HR isdriven with increased output, and the left-hand rear wheel HL is brakedto maintain constant revolutions. Additionally, with pump 21 switchedon, a changeover valve 20 is also required in order to be able to supplythe right-hand front wheel VR with brake pressure. In other respects,the parts in FIGS. 1 and 2 correspond to one another with parts DBN,USV1 and LV of FIG. 2 corresponding to parts 3, 2 and 7, respectively,of FIG. 1.

FIG. 3, shows the control circuit for the system of FIG. 1, this controlcircuit is designated 30. It is connected with four sensors 31 forregistering the wheel speeds, with four ABS valves 4', 6' and 8'(corresponding to 4, 6 and 8 of FIG. 1), three changeover valves 2', 5'and 9', two pumps 1' and 11', and an actuator 32 for a throttle valve33.

In the ABS case, two pumps 1' and 11' are in operation and the brakepressure is varied by means of the valves 4', 6' and 8', in accordancewith the wheel movement behavior monitored via the sensors 31.

In the ASR case, the valves 2' and 9' are changed over and both pumps 1'and 11' are switched on. Pressure is varied on the front wheels by meansof the valves 4' and 8'. In addition, the throttle valve 33 is adjusted,via the actuator 32 so as to reduce engine torque.

In the case of the measurement of μ, the valves 2' and 5' are switchedand both pumps 1' and 11' are running. By means of the valve 6, thebrake pressure at the wheel HR is pulsed up. Simultaneously, controlledby the control device via the throttle valve actuator 32, the enginetorque is increased so that the braking force of the wheel HR and thedriving force of the wheel VR equal each other. The algorithm forcontrolling the driving force and the braking force must be designedspecific to the vehicle. Programming takes place in the computer of thecontrol device. Additionally, the wheel VL is now braked in such a waythat despite increased driving force, its revolutions do not increase,but are maintained, for example, at the same revolutions of the wheelVL.

The number of pulses required to create the tendency to lock at thewheel VR is a measure of the desired coefficient of friction μ, and thiscan then be indicated (display 34) after conversion, and/or trigger awarning (warning light 35).

I claim:
 1. A method for determining a coefficient of friction μ betweena tread of a tire of a vehicle, having at least two driven and at leasttwo non-driven wheels, and a road surface, using pressure controldevices for application of respective brake pressure at wheel brakes,and using control circuit, which receives signals corresponding to wheelspeeds, for controlling the pressure control devices and for varyingengine torque, whereinin the case where the vehicle is not being brakedand is not significantly accelerated, the control circuit continuouslyincreases, in response to a trigger signal at a start time, and by meansof the pressure control devices, a brake pressure which produces abraking force on a measuring wheel which is not driven; simultaneously,the control circuit increases engine output to increase a driving forceon a first driven wheel on the same side of the vehicle as the measuringwheel such that the braking force of the measuring wheel and the drivingforce of the first driven wheel approximately compensate each other; asecond driven wheel is braked such that it remains approximately at agiven speed; and a measuring time period required for increasing thebrake pressure to produce locking of the measuring wheel is determinedand used as a measure for the coefficient of friction.
 2. A method inaccordance with claim 1, wherein the braking pressure at the measuringwheel is produced by braking pulses, and the measuring time period isdetermined from the number of braking pulses.
 3. A method in accordancewith claim 1, wherein the given speed of the second driven wheel isapproximately the same as a speed of a non-driven wheel on the same sideof the vehicle.
 4. A method in accordance with claim 2, wherein theincreasing in the driving force on the first driven wheel takes place insteps.
 5. A method in accordance with claim 1, wherein the coefficientof friction is indicated.
 6. A method in accordance with claim 1,wherein when the coefficient of friction is below a threshold, at leastone of an acoustic and a visual warning is given.
 7. A method inaccordance with claim 1, wherein the trigger signal is produced by adriver.
 8. A method in accordance with claim 1, wherein the triggersignal is produced automatically.
 9. A method in accordance with claim1, wherein the given speed of the second driven wheel is approximatelythe same as a speed of the second driven wheel at the start time.
 10. Amethod for determining a coefficient of friction (μ) between a tread ofa tire of a vehicle, having at least two driven and at least twonon-driven wheels, and a road surface, in the case where the vehicle isnot being braked and not significantly accelerated, using pressurecontrol devices for application of brake pressure at respective wheelbrakes, and using a control circuit, which receives signalscorresponding to wheel speeds, for controlling the pressure controldevices and for varying engine torque, the method comprising the stepsof:generating a trigger signal at a start time; causing, in response tothe trigger signal, a continuous increase in a brake pressure whichproduces a braking force on a measuring wheel which is a firstnon-driven wheel, and causing a simultaneous increase in the enginetorque to increase a driving force on a first driven wheel on the sameside of the vehicle as the measuring wheel such that the braking forceof the measuring wheel and the driving force of the first driven wheelapproximately compensate each other; braking a second driven wheel suchthat it remains approximately at a given speed; determining a measuringtime period required for increasing the brake pressure to producelocking of the measuring wheel; and calculating the coefficient offriction based on the measuring time period.
 11. A method in accordancewith claim 10, wherein said step of causing includes producing thebraking pressure at the measuring wheel by braking pulses, and whereinsaid step of determining includes determining the measuring time periodfrom the number of braking pulses.
 12. A method in accordance with claim11, wherein said step of causing includes increasing the driving forceon the first driven wheel in steps.
 13. A method in accordance withclaim 10, wherein said step of braking includes causing the given speedof the second driven wheel to be approximately the same as a speed of asecond non-driven wheel on the same side of the vehicle.
 14. A method inaccordance with claim 10, further comprising a step of indicating thecoefficient of friction.
 15. A method in accordance with claim 10,further comprising a step of determining whether the coefficient offriction is below a threshold, and a step of providing at least one ofan acoustic and visual warning if the coefficient of friction is belowthe threshold.
 16. A method in accordance with claim 10, wherein saidstep of generating includes a vehicle driver producing the triggersignal.
 17. A method in accordance with claim 10, wherein said step ofgenerating includes producing the trigger signal automatically.
 18. Amethod in accordance with claim 10, wherein said step of brakingincludes causing the given speed of the second driven wheel to beapproximately the same as a speed of the second driven wheel at thestart time.